Does (display) mean that the card is not fully utilized for Cycles rendering?
Calculating and refreshing all the pixels on the screen(s) is the main job of a GPU and this requires a significant amount of resources, which will be scarce when the GPU is used for rendering as well.

So if you want your display to be responsive while GPU-rendering, then it's a good idea to reserve one GPU just for its originally intended job of of pushing those pixels to the display(s).

The '(Display)' text from the CUDA settings helps you accomplish this, by indicating which of your GPUs is currently connected to the display(s).
Otherwise you wouldn't be able to tell which one is which if you had say 3 x GTX 1070.